1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method that perform pseudo-halftone processing for multivalued image data.
2. Description of the Related Art
In general, a digital image processing apparatus such as a digital printer including a laser beam printer is widespread as a consequence of development of digital devices, in substitution for a conventional analog image processing apparatus. The digital image processing apparatus usually adopts a method for gradation reproduction with halftone processing such as a dither method for reproducing a halftone.
Above mentioned method is used for a portion, e.g., a flat portion, which consists of a small amount of high frequency components. However, in an image having a periodic pattern such as halftone dots or a character/thin line portion, a dither periodic pattern interferes with a high frequency component (particularly, a periodic pattern close to that of a dither period) included in an input image. Accordingly, periodic fringe pattern (hereinafter, referred to as moire), i.e., a moire phenomenon is generated.
In contrast, in a portion, e.g., a flat portion showing a small amount of high frequency components, in which a gradation expression can be achieved using the dither method, pseudo-halftone processing using the dither method is employed. On the other hand, in a portion, e.g., a character or thin line portion showing a large amount of high frequency components, an input image is converted into an analog signal without the dither processing, and the converted signal is compared with a periodic pattern signal and is then subjected to image processing of pulse width modulation. As described above, with a method discussed in Japanese Patent Application Laid-Open No. 7-154599, processing contents are switched depending on a threshold (image pattern).
However, in processing for switching between an image that is subjected to the dither processing and an image that is not subjected to the dither processing depending on the threshold, at a switching portion of the processing, the dot structure extremely changes. Therefore, particularly, in a natural image, the strangeness can be felt at the switching portion of the processing.